An optical waveguide type signal light control device such as an optical modulator, an optical switch, etc. has been considered as a key element in an optical high speed communication system, an optical information processing system, etc., and the research and development thereof has been intensively active these days. As the optical waveguide type signal light control device, there are ones using a dielectric material such as LiNBO.sub.3, etc. and the others using a semiconductor such as InP, GaAs, etc. Of the two kinds of optical waveguide type signal light control devices, the latter type is expected to be widely used as an optical semiconductor modulator, an optical semiconductor switch, etc., because it can be integrated with other optical devices such as an optical amplifier, etc. and an electronic circuit of FET, etc. to make the structure small and a voltage to be used therein low. For instance, such an optical modulator is described in Japanese Patent Kokai No.3-44618.
In a conventional InP system waveguide type signal light control device such as an optical modulator, an optical switch, etc. which is used in an optical communication system, an optical information processing system, etc. using a signal light of 1.3 and 1.55 .mu.m bands, an guiding layer, an absorption layer, or an active layer for confining light comprises multi-quantum wells of InGaAsP mixed crystal, or this mixed crystal and InP, and cladding layers for sandwiching the guiding layer, the absorption layer, or the active layer is only of InP.
In the conventional waveguide type signal light control device, however, a radiation angle of light vertical to layers becomes large, because a refractive index difference is large between InGaAsP, especially, of a wavelength composition proximate to 1.55 .mu.m and InP. Therefore, a large coupling loss occurs between the signal light control device and a single mode optical fiber. For reducing the large coupling loss, it is considered to decrease the radiation angle by decreasing a thickness of the guiding layer, the absorption layer or the active layer. In such a case, however, there is a disadvantage in that an operation voltage becomes high, because the confinement of light is deteriorated in the guiding layer, the absorption layer or the active layer.
In fabricating the conventional waveguide type signal light control device of InP system, a ridge portion or a mesa portion is prepared in either of a ridge type or a buried hetero type by using wet chemical etching. In this method, however, it is difficult to form a fine waveguide on a large area with high reproducibility. Further, side walls of the ridge or mesa portion are not of flat crystal planes, and a width thereof is deviated. As a result, scattering light loss is increased, and field pattern of a radiation light is not smooth, so that a coupling loss becomes large between the device and an optical fiber.
On the other hand, when dry etching is used, a fine waveguide is formed on a large area with high reproducibility. However, it can not be avoided in the presently available dry etching technology that minutely convex and concave surfaces are found on etching bottom and side planes to result in the occurrence of scattering loss for propagation lights. Further, the dry etching damages the etching bottom plane to also result in the occurrence of loss for the propagation lights. For there reasons, it is difficult to decrease the light loss in an optical semiconductor waveguide fabricated by using dry etching to an extent to that in an optical semiconductor waveguide fabricated by using wet chemical etching. Even worse, when a material is of InP system, there is a further disadvantage in that dry etching per se is not completed in technology.